Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

How to make AM radio transmitter with only BJTs

Fluffyboii

Active Member
I found this beautiful clock/radio with turquoise vacuum display (it looks more blue on camera). It required the regular cleaning of pots and switches and for some reason someone didn't solder its speaker. Only problem now is that there are no good radios in range here. The antenna of this thing was cut so I soldered a new wire to make it longer but idk how long it should be, at least it gets better signal now. If someone can translate what switches mean it would be nice, for example what AUTO MANUAL or UHR and AUS mean. I pretty much figured the rest.

MW gets some radios with regular fiddling of the radio at 3AM but not as well as my ferrite core antenna radio.
This really made me want to make a simple AM radio and play my own music on it. My school has many powerful transistors in its storage that I can use like bc140, bc160, there are also very overpowered transistors with metal casing that acts like one of the connections. It is really stupid that they got those very expensive BJTs but didn't get any comparators, 555 ICs and other simple stuff that students often need.

The thing is I accidentally made the required components of an radio for my modular synth with op amps. They wont work at high frequencies but fun to play with for making music. I also have a vague idea of what I need to make one from my analog/digital communication class. I need a pure sine wave fed to a voltage controlled amplifier that is biased with some DC voltage so that it can be demodulated with a simple diode envelope.

I could probably use a simple op amp amplifier for sound input stage and add a electret mic for fun but I am really in the mood of torturing myself with analog circuit design. I want to make something I can show to my teachers with pride that only consists transistors soldered on a piece of copper like how old electronics predating PCBs were put together.
I was looking at different circuit designs for simple AM radios. Most of them are using base of a BJT to modulate the signal by changing the collector current and a simple RC circuit to determine the operating frequency. Or there are circuits that I don't understand anything about because they are hella complex.

For sound input stage I think I can get away with a simple CS amplifier and a source follower to amplify the input since I don't need to much gain and I don't think inverted signal will prove any problems. The confusing thing is getting a pure sine wave with a fixed frequency. I was thinking of using a quartz crystal but smallest I can get is 2Mhz and even if I use a clock divider I don't think it is wise to use 1Mhz and I should use a less used frequency at the edges of MW radio frequencies. There was a LM13700 triangle to sine shaper circuit I made for an LFO but even though sine wave out of that thing looked nice I don't think it can operate in Mhz range. Looks like I need to use a simple BJT RC oscillator. Single transistor ones seems a bit unreliable so I will need to look into it. I also need to figure out a way to make a transistor VCA. This one is nice but idk how to make it without op amps since its is using one to subtract voltages and it probably doesn't work with negative CV inputs. One way I know to make a multiplier is the simple 2 transformer, 4 diode textbook sample but those transformers are expensive and hard to deal with in audio frequency range and I expect non the less for AM range.
1669645629235.png

Before jumping to say transmitting radio signals are illegal, I know. As I said there are no AM radios here that can be heard with even the best radio I could find in daytime. At the middle of the night I can find some Russian or Greek radios and thats it. And I also know that doesn't change the fact that this thing can create interference and alarm some authorities. All I want to make a nice transmitter that can be scaled up and doesn't use sketchy techniques to keep part number or complexity low and learn a bit more about radio transmission stuff. I will eventually figure it out myself by checking old radio books and textbooks. I think if I can make something nice it would at least not create unnecessary interference and it is not like I will transmit radio 7/24.
 

Attachments

  • IMG_20221128_002239.jpg
    IMG_20221128_002239.jpg
    387.6 KB · Views: 50
  • IMG_20221128_003026.jpg
    IMG_20221128_003026.jpg
    499.6 KB · Views: 51
Last edited:

JimB

Super Moderator
Most Helpful Member
Like the modulator I did build a while ago which does output an amplitude modulated signal:
1670253252424.png
That looks like the basis of a balanced modulator, you are likely to get a double sideband supressed carrier signal out of that, rather than a correct amplitude modulated carrier.

To modulate your oscillator, I suggest a method of varing the supply voltage as per the circuit which you showed earlier:

1670261808877.png

I have highlighted in red the part of the circuit which is intended to vary the supply voltage to the oscillator.
However, I am not convinced that this particular circuit will work very well.
There are circuits which could work better.

JimB
 

Fluffyboii

Active Member
That looks like the basis of a balanced modulator, you are likely to get a double sideband supressed carrier signal out of that, rather than a correct amplitude modulated carrier.

To modulate your oscillator, I suggest a method of varing the supply voltage as per the circuit which you showed earlier:

View attachment 139536
I have highlighted in red the part of the circuit which is intended to vary the supply voltage to the oscillator.
However, I am not convinced that this particular circuit will work very well.
There are circuits which could work better.

JimB
Yes, you are right.
I like the first circuit at #11, I feel like I can replace the crystal oscillator with the one I made. I tried changing supply voltage of the oscillator to see what happens, it seems like it can be done to get AM modulation.
 

Fluffyboii

Active Member
I test the coil. Decreased capacitances and added an adjustable capacitor in parallel. Will test new freq tomorrow.
 

Attachments

  • IMG_20221205_211857.jpg
    IMG_20221205_211857.jpg
    7.3 MB · Views: 31

Fluffyboii

Active Member
I did make it work.
I used the first schematic from #11 but instead of crystal oscillator I used the one I made. I seperated the oscillator power and connected a 6V linear voltage regualtor to it to feed it separately to decrease its output voltage. I had two small inductors from a CRT TV that had too much inductance when I used one of them but two in parallel comes up as 25uH which according to expalation of the schematic is perfect. Adding tank capacitor across it around 30pF according to the graph did only decrease output voltage so I ignored the 30pF capacitor. I used single stage bjt amplifier for amplifying my phone jack output and used bypass capacitors to increase gain just enough to have some what ideal modulation. I will draw the whole diagram. I still did not get close to the voltage I simulated with LTspice. I guess my ghetto hand work created a lot of stray capacitances. With random alligator cable as antenna, range is less than 5 meters so I guess it is not a electromagnetic hazard. Frequency that it oscillates is around 1.2Mhz. I am thinking of cutting the excess ferrite rod to decrease inductance a little to increase freq. Electret mic required more gain than I thought and I wasn't able to get it with simple common source amplifiers so I gave up on it, it is not connected to anywhere.
 

Attachments

  • IMG_20221207_023505.jpg
    IMG_20221207_023505.jpg
    1.7 MB · Views: 27
  • IMG_20221207_023910.jpg
    IMG_20221207_023910.jpg
    1.6 MB · Views: 25
  • IMG_20221207_024052.jpg
    IMG_20221207_024052.jpg
    2.3 MB · Views: 28
  • AM modulator.asc
    3.2 KB · Views: 21
  • Screenshot_1.png
    Screenshot_1.png
    144.9 KB · Views: 28
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
Wind the coil on a thin cardboard former, wrapped around the ferrite rod - this allows you to slide the coil along the rod, and adjust the inductance. This is how ferrite rod based radios were (and still are) adjusted - if I remember correctly? (I've not done one for over 45 years), you adjust the inductance at the low frequency end, and the capacitance (with a trimmer cap) at the high frequency end.
 

Fluffyboii

Active Member
Wind the coil on a thin cardboard former, wrapped around the ferrite rod - this allows you to slide the coil along the rod, and adjust the inductance. This is how ferrite rod based radios were (and still are) adjusted - if I remember correctly? (I've not done one for over 45 years), you adjust the inductance at the low frequency end, and the capacitance (with a trimmer cap) at the high frequency end.
I will note that for future builds. What do you think about the lower than expected performance of this transmitter. I wonder if the small torroid inductors have bad Q factor or something like that that makes them unsuitable for this application. I also noticed that I wasn't able to reach %100 modulation with 2V peak to peak input signal. There is something that is causing the modulation to be weak and transmitter to be under powered. I am not sure if it is worth investigating.

I actually noticed that oscilloscope probe is loading down the signal quite a lot. So simulated 30V peaks vs measured 10V may make sense. Also probe is probably adding some capacitance as well. So I added the 30p capacitor to LC circuit. I used my laptop as sound source since it can get more output voltage and that way I was able to get close to %100 modulation and volume output from radio dramatically increased.

RF stuff is damn complicated, can't imagine how hard it would be to make something run at 10Mhz and can not comprehend how people design stuff for 5Ghz.

The radio zapped me guys. Inductors actin like boost converters. It zapped me through the body of the linear converter, It seems like positive side is charged with high voltage. Is that normal.

A better radio perhaps could be created with some high GBW op amps. Want to make a Wien bridge to get a pure sine wave and make a VCA for high frequencies. That could be perhaps amplified without creating havoc on other frequencies.
 
Last edited:

Latest threads

New Articles From Microcontroller Tips

Top